drm/i915/guc: Handle context reset notification

GuC will issue a reset on detecting an engine hang and will notify
the driver via a G2H message. The driver will service the notification
by resetting the guilty context to a simple state or banning it
completely.

v2:
 (John Harrison)
  - Move msg[0] lookup after length check
v3:
 (John Harrison)
  - s/drm_dbg/drm_err

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-13-matthew.brost@intel.com